This invention relates in general to an efficient display device capable of displaying monochromatic, multi-color and full-color images of high brightness and resolution. Specifically, the invention relates to a liquid crystal device (LCD) without color filters, where the LCD is illuminated by a back lighting source, which is an electronic fluorescent source emitting monochromatic light or light of multiple colors, such as the three primary colors of red, blue and green.
LCDs are one of the most widely used type of devices. However, most of the LCDs used today are monochromatic. While multi-color and full-color LCDs have been proposed, their development has been hindered by a number of technical difficulties. In most of the multi-color and full-color LCDs proposed, a back light source is employed. However, in most cases, the back light source employed is white light. Therefore, to produced composite images of different color, red, blue and green filter arrays have been used. For each pixel, the white light directed towards a portion of the pixel is filtered to permit only red light to pass, and white light directed toward another portion of the same pixel is filtered to permit only blue light to pass and the white light directed towards the remaining portion is filtered to permit only green light to pass. Thus only a small part of the energy of the white light is transmitted through the LCD. If relatively pure red, blue and green light is desired, the filters employed must have narrow pass bands, so that the percentage of the energy of the white light utilized is further reduced. Alternatively, if a brighter display is desired, the user may have to compromise on the color quality and utilize red, blue and green filters with broader pass bands.
LCD cells respond slowly to voltages applied across them. Typically, when scanning voltages are first applied to a LCD cell, the cell has low transmission rate. The transmission rate rises slowly during the scanning cycle so that a low percentage of light is passed by the red, blue and green filters and transmitted through the LCD cells during the scanning cycle. This is a notable drawback of passive matrix type LCD color displays, where no drivers are used contiguous to the LCD cells for driving the cells.
To improve display brightness, active matrix LCD cells are proposed by adding at least three thin film transistors for each LCD cell or pixel for accelerating the turning on and off of the three portions of the cell or pixel for light transmission of the three different colors. Such transistors, however, are opaque and occupy a significant area of the LCD cell. In other words, whatever the designer may have gained by increasing the transmission rate, the designer will lose at least part of the advantage because of the reduction of the area of the cell that actually transmits light.
A further complication in the active matrix LCD type displays is in manufacturing. Thus if a thin film transistor in one of the LCD pixels or cells is defective, the entire display is useless and must be discarded. Because of yield problems, redundant transistors are implemented. However, adding more thin film transistors further reduces the light transmitting portion of the pixel and is undesirable. For a display with many pixels, the reduction in area is considerable. For example, for a 480 by 240 pixel display, 480.times.240.times.3 transistors must be used even without any redunduncy in transistors. If redundunt transistors are included, such as by using two transistors for each color in a pixel, 480.times.240.times.3.times.2 transistors must be used.
For the reasons above, it is difficult to use the above-described conventional designs to achieve efficient color LCD displays of high brightness, good color and high resolution. This is particularly the case for large displays. It is therefore desirable to provide an alternative design for color LCD displays which are inexpensive and where the above-described difficulties are avoided or alleviated.